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I am running raspibian Jessie on raspberry pi 3,
And I want to backup the entire raspibian OS to a disk image!
Is it possible if I can backup to disk right from within raspibian OS running installed from the sd card on raspberry pi3?
You could, if you really want an image with lots of rubbish. For a more practical approach see can-a-raspberry-pi-be-used-to-create-a-backup-of-itself